Keep Go's build cache between builds for make test (closes #124)

The Dockerfile runs make test with Go's build cache in a BuildKit cache
mount, so a build compiles only what changed since the last one instead
of the standard library and every dependency from nothing. script/test
passes -count=1, so no test result is taken from the cache.

Model: opus-5-5

Dockerfile

@@ -50,7 +50,11 @@ ARG CHECK_EPOCH
 
 COPY . .
 
-RUN make test
+# Go's build cache is kept between builds in this cache mount, so make test
+# compiles only what changed since the last build, not the standard library
+# and every dependency from nothing. script/test passes -count=1, so test
+# results are never taken from it.
+RUN --mount=type=cache,target=/root/.cache/go-build make test
 
 # The version stamped into the binary: the VERSION build argument when one
 # is given, otherwise `git describe --tags --always` of the .git the build